1. Field of the Invention
This invention relates generally to radiographic apparatus and more particularly to apparatus for removing x-ray scatter during radiographic imaging.
2. Description of the Prior Art
When one exposes an object, such as a patient, to x-rays during a diagnostic imaging procedure, the x-ray tube assembly acts like a point source whereupon x-rays are emitted in a straight line. They then pass through the patient and impinge on a detector, which in the case of fluoroscopy, comprises electronic imaging apparatus such as an image intensifier coupled to a television camera. The x-rays that pass through the patient in a straight line are known as primary x-rays and are those desired to be detected. Other x-rays, known as scatter, in passing through the patient emerge at widely different divergent angles and are normally suppressed insofar as possible. There are a number of existing techniques for discriminating between primary and scattered radiation. The most common means is to utilize an anti-scatter grid which is placed behind or under the patient and in front of the detector. The anti-scatter grid is comprised of, for example, a series of lead or tantalum or other heavy metal strips that are laminated to form a plurality of parallel septa which permit x-rays to pass through if they are more or less parallel to the septa but x-rays which pass through at divergent angles have to pass through the lead or heavy metal and are thus attenuated. Disadvantages of an anti-scatter grid are several; first, it is far from perfect and still permits some scattered radiation to reach the detector. Second, a portion of the primary radiation also is removed, meaning that the total amount of radiation delivered to the body undergoing a radiographic procedure must be increased to compensate for that lost in the anti-scatter grid, meaning that whenever the primary radiation is attenuated or scattered radiation is detected, it is necessary to increase the x-ray dose to the body in order to obtain the same statistical information out of the image. Thus a trade off has normally existed between the loss of primary radiation, the x-ray dose and the removal of scatter.
Another known method that has been used to remove x-ray scatter employs a pair of thin slits, constructed of strongly x-ray attenuating material. One slit (foreslit) is placed between the x-ray source (tube) and the object (patient) being imaged. The second, a larger slit (aftslit) is placed behind or beneath the object (patient) and ahead of the detector. The slits are precisely aligned so that x-rays that pass in a straight line from the source and through the foreslit, also will pass through the aftslit if the x-rays are not scattered within the object (patient). X-rays that are scattered within the patient are most likely to strike the x-ray absorbing material contained within the aftslit. An image of an extended object is made by scanning the x-ray source, foreslit and aftslit in synchronism. Additionally, a plurality of paired slits have also been used. This method of scanning paired or multiple paired slits is intrinsically more efficient in rejecting scattered radiation than the use of an anti-scatter grid, but also is more complicated. A significant disadvantage of this method is that due to the relatively great weight and bulk of the aftslit, scanning and alignment of the paired slit assemblies present a substantial mechanical problem. Additional space is also required between the patient and the image receptor for the aftslit assembly, which causes undesirable magnification of the image and, usually, a specially constructed or modified patient support table is needed.
Accordingly, it is an object of the present invention to provide an improvement in apparatus for making a radiographic image.
It is another object of the invention to provide an improvement in the ratio of the primary radiation to the scatter radiation in an image generated during a diagnostic imaging procedure.
It is a further object of the invention to remove the anti-scatter grid assembly normally located between an object receiving x-ray radiation and an x-ray detector.
It is still another object of the invention to electronically collimate x-ray radiation for removing scatter during a radiographic imaging procedure.